An RF power amplifier provides the final stage of amplification for a communication signal that has been modulated and converted into an RF signal. Often that RF signal exhibits frequencies in a predetermined frequency band that is licensed by a regulatory agency for a particular use. The RF power amplifier boosts the power of this RF communication signal to a level sufficient so that the signal, when it propagates to an antenna, will be broadcast in such a manner that it will meet the communication goals of the RF transmitter.
Many popular modern modulation techniques, such as CDMA, QAM, OFDM, and the like, require the RF power amplifier to perform a linear amplification operation. In other words, the RF communication signal conveys both amplitude and phase information, and the RF power amplifier should faithfully reproduce both the amplitude and phase content of the RF signal presented at its input. While perfect linearity is a goal for any linear RF power amplifier, all linear RF power amplifiers invariably fail to meet it. The degree to which the goal of perfect linearity is missed leads to unwanted intermodulation, nonlinearities, and spectral regrowth.
The regulatory agencies that license RF spectrum for use by RF transmitters define spectral masks with which transmitters should comply. The spectral masks set forth how much RF energy may be transmitted from the RF transmitters in specified frequency bands. As transmitter technology has advanced, and as increasing demands have been placed on the scarce resource of the RF spectrum by the public, the spectral masks have become increasingly strict. In other words, very little energy outside of an assigned frequency band is permitted to be transmitted from an RF transmitter. Accordingly, unless the spectral regrowth that results from any nonlinearity in the amplification process is held to a very low level, the RF transmitter will be in violation of its regulatory spectral mask.
In conventional RF transmitters, the amplifier linearity requirement is usually difficult to achieve at a reasonable cost. In general, more sophisticated and expensive amplifiers can be devised which exhibit better linearity. But always, cost is desirably minimized, and the minimization of cost is particularly important for mass market devices, such as cell phones, tablet devices, and other handheld devices, that include RF transmitters. In many applications, the poor linearity of a low cost amplifier may be made acceptable through the use of pre- or post-amplification distortion cancelation, compensation or linearizing techniques that lead to cost improvements when compared to the use of sophisticated and expensive amplifiers.
In conventional RF transmitters, the amplifier linearity and cost parameters are counterbalanced against power-added efficiency (PAE). Power-added efficiency is the ratio of the RF output power to the sum of the input RF power and the applied bias-current power. An amplifier that has low PAE wastes power, which is undesirable in any transmitter, but particularly undesirable in battery-powered transmitters because it necessitates the use of undesirably large batteries and/or undesirably frequent recharges. Conventionally, improvements in PAE have been achieved at the expense of linearity.
Another factor that affects costs, linearity, and PAE is an RF amplifier's dynamic range. A peak of a communication signal represents the greatest instantaneous amplitude, magnitude, or power level exhibited by a communication signal within some period of time. An amplifier that is required to have a large dynamic range and to faithfully reproduce a communication signal with occasional large peaks also tends to be more expensive and exhibit less PAE than amplifiers that are not required to have such a large dynamic range. And, if the amplifier simply does a poor job of reproducing the peaks, then linearity suffers. From another perspective, an RF amplifier with a smaller dynamic range may be a used if a communication signal is attenuated so that its occasional large peaks fit within the smaller dynamic range. But this causes the average power level to be reduced, thereby reducing link margins and reducing the amount of data that may be communicated over the link.
To address these competing RF amplifier design considerations, conventional transmitters have added various circuits to compensate for the shortcomings of a less expensive RF amplifier. One such circuit is dynamic amplifier bias control, which may improve PAE. While various forms of dynamic amplifier bias control are known, an envelope-tracking technique has particularly desirable attributes. Envelope tracking provides a bias control signal that roughly follows the envelope of the RF communication signal, but does not completely follow the envelope. The envelope tracking technique generates the amplifier bias control signal to exhibit a significantly lower bandwidth than the RF communication signal, but to nevertheless track the RF communication signal's magnitude peaks. One example of an envelope tracking form of dynamic amplifier bias control is described in U.S. Pat. No. 7,570,931, issued 4 Aug. 2009, and entitled “RE Transmitter With Variably Biased RF Power Amplifier And Method Therefor,” which is incorporated by reference in its entirety herein.
The lowered bandwidth lowers the switching frequency requirements in the power supply that generates the bias voltage applied to the RF power amplifier's power input. In theory, accurately following the RF communication signal's envelope over a higher bandwidth would achieve greater PAE improvements, but in practice it would require the use of such an expensive power supply that any cost savings in the RF amplifier would be lost. Moreover, a large amount of power is likely to be consumed by such a higher bandwidth power supply, and additional unwanted RF noise is likely to be generated. The use of a lowered bandwidth amplifier bias control signal permits the use of a low power, low noise, low cost power supply that can nevertheless achieve significant PAE improvements.
Unfortunately, dynamic amplifier bias control does nothing to lessen dynamic range constraints for the RF amplifier. Thus, an alternate circuit that conventional transmitters have devised to address the competing RF amplifier design considerations and compensate for the shortcomings of a less expensive RF amplifier is a peak-to-average-power-ratio (PAPR) reduction circuit. An average of the communication signal represents the average amplitude, magnitude, or power level of the communication signal over a given period. The peak is greater than the average, and the ratio of the peak power to the average power (PAPR) is a parameter of interest to communication system designers.
One example of a PAPR reduction circuit is described in U.S. Pat. No. 7,747,224, issued 29 Jun. 2010, and entitled “Method and Apparatus For Adaptively Controlling Signals”, which is incorporated by reference in its entirety herein. A PAPR reduction circuit like the one described in U.S. Pat. No. 7,747,224 and elsewhere, reduces the communication signal peaks prior to amplification, thereby reducing dynamic range constraints on the amplifier. And, by reducing the largest peaks of the communication signal, the biasing voltage for the RF amplifier may be reduced, thereby improving PAE at the same time. Most linear power amplifiers become more power efficient as the PAPR decreases. And, other benefits come from operating transmitters at a lower peak but greater average power, such as increasing link margins and permitting greater amounts of data to be transmitted in a given period of time.
The reduction of communication signal peaks in a PAPR reduction circuit, also referred to below as a peak reduction (PR) circuit, introduces noise into the communication signal, but the PR circuit is desirably configured so that this noise is primarily located in-band and so that no spectral mask violations occur. The transmitted in-band noise is often characterized using an error-vector magnitude (EVM) parameter. EVM specifications are based upon achieving a desired signal-to-noise ratio (SNR) at a receiver for a given modulation order and coding rate. EVM may be designated as the ratio of the total amount of noise power in a communication signal to the total signal power in that signal. It is usually specified as a percentage, equal to one-hundred divided by the square-root of the SNR.
In the version of a PR circuit described in U.S. Pat. No. 7,747,224, a signal magnitude threshold, which defines the level of the peaks in a reduced-peak version of the communication signal, may be controlled to maintain the EVM parameter precisely at a maximum amount allowed by the transmitter's specifications. In other words, if an EVM specification allows the transmitter to transmit more in-band noise, then the transmitter spends some of its available EVM budget in order to get improved PAE and link margins.
FIG. 1 shows a chart that graphically depicts the operation of this PR circuit with respect to a defined threshold. An inflated-peak communication signal 10 is presented to the peak reduction circuit (not shown), depicted as the magnitude of signal 10 in FIG. 1. Communication signal 10 includes a local peak 12 at the apex of an excursion portion 14 that extends above a signal magnitude threshold 16. The goal of the peak reduction circuit is to remove excursion portion 14, thereby reducing the magnitude of local peak 12 and causing the magnitude of a reduced-peak communication signal to follow the dotted line trajectory 18 in the vicinity of excursion portion 14 rather than to include excursion portion 14. Desirably, excursion portion 14 is removed in a manner that primarily introduces in-band distortion into the reduced-peak communication signal, when compared to the inflated-peak communication signal.
Signal magnitude threshold 16 defines the maximum peak values achieved in the resulting reduced-peak communication signal. By increasing signal magnitude threshold 16, less peak reduction results, less power-added efficiency (PAE) is achievable in a downstream RF power amplifier, and a lower average power output is available from the RF power amplifier. But, less noise is introduced into the communication signal. By decreasing signal magnitude threshold 16, a greater amount of peak reduction results, more power-added efficiency (PAE) is achievable in the RF power amplifier, and a higher average power output is available from the RF power amplifier. But, these beneficial power amplifier consequences come at the cost of introducing more noise into the communication signal.
This prior art peak reduction circuit contemplates the possible use of a variable signal magnitude threshold 16. In particular, an error-vector magnitude (EVM) indicator (not shown) or other control indicator may be used to adjust signal magnitude threshold 16 by increasing and decreasing so that noise power is held roughly constant, slightly below the maximum EVM permitted for the communication system of which the peak reduction circuit is a part.
But the EVM or other control indicators are deeply lagging and slowly varying indicators. As a deeply lagging indicator, the indicator is responsive to a portion of the communication signal that occurred over some past period in time compared to the current state of the communication signal. And that past period in time typically occurred far in the past, at a delay greater than the latency of all remaining circuits in the transmitter downstream of the peak reduction circuit. It is not a precise indicator of the current state of the communication signal being processed in the PR circuit. The EVM indicator, for example, is slowly varying because it is formed by accumulating instantaneous indications obtained by processing the amplifier's output signal over a considerable duration. As a consequence, the conventional lagging-indicator signal magnitude threshold signal 16 appears to be virtually invariant over the duration of excursion 14 and even over the entire time reflected in FIG. 1.
Unfortunately, conventional PR circuits with lagging-indicator signal magnitude threshold signals tend to reduce peaks in a way that is too imprecise and is largely incompatible with dynamic amplifier bias control. In other words, power savings and PAE improvements may be achieved through dynamic amplifier bias control, and similar improvements may be achieved by using a PR circuit, but using both conventional dynamic amplifier bias control and a conventional PR circuit tends to introduce no significant further improvements to those achievable using either one alone.